This section provides background information related to the present disclosure which is not necessarily prior art.
With the ongoing urban security construction, the intelligent traffic industry is developing rapidly. Due to the need for capturing moving objects (e.g., vehicles) and identifying features of the moving objects (e.g., license plates and colors of vehicles), high requirements are placed on smart cameras used in traffic systems (i.e., smart traffic cameras).
The requirements for smart traffic cameras may include, for example, instant solidation capability to minimize deformation of moving objects and the capability of producing images with adequate brightness and good quality due to the need for identifying features of moving objects, which requires ample light.
Conventional smart traffic cameras may be implemented using charge coupled device (CCD) sensors together with light emitting diodes (LED) for light.
FIG. 1 is a diagram illustrating a conventional exposure mode using a CCD sensor and a conventional light mode using an LED. As shown in FIG. 1, the CCD sensor uses a global exposure mode. Exposure of all pixels in a frame image starts and ends at the same time. Charges in the pixels are transferred using a vertical charge coupled device (VCCD) and light is provided by an LED having a certain duty cycle. Supposing the readout time of each frame image (Tf) is 40 ms, i.e., the frame rate of the CCD sensor is 25 fps, and the exposure time (Ts) is 5 ms, an LED having a duty cycle of 12.5%, a high-light time of 5 ms, and a low-light time of 35 ms may be used for providing the light to each frame image. N represents the N'th frame image.
According to the above mechanism, there is little deformation of moving objects and images captured have good brightness and good quality.
CCD sensors are expensive due to the restrictions of the production process, thus, smart traffic cameras equipped with CCD sensors are very expensive, which hampers the mass deployment of such smart traffic cameras.
Therefore, another mechanism for implementing smart traffic cameras was proposed, which uses complementary metal oxide semiconductor (CMOS) sensors and constant-lit LED for light.
FIG. 2 is a diagram illustrating a conventional exposure manner of a CMOS sensor. As shown in FIG. 2, the CMOS sensor uses an exposure manner of rolling shutter, i.e., reset and readout of charges are carried out at the same time. For each row of pixels, shutter is the time difference between the start time of resetting charges (reset line) and the start time of reading out the charges (read line). For this exposure method, the light is usually provided to the whole frame image to ensure that the whole frame image is uniformly lit.
The above mechanism, however, has some deficiencies in practice. For example, the light is power-consuming, fragile, and generates excessive heat because it is constantly switched on, which impairs the light performance. Further, taking CMOS sensors having a frame rate of 30 fps as an example, under the exposure mode of rolling shutter, the time difference between the readout time of the top of a frame image and the readout time of the bottom of the frame image is the readout time of the frame image, i.e., T=1/30 s=33.3 ms. The large time difference results in large deformation of moving objects.